Dental caries is the most prevalent infectious and chronic disease affecting humans, and is associated with costly treatment worldwide. The transition from dental health to dental caries is characterized by compositional and metabolic changes in the complex microbial communities of oral biofilms. Oral biofilms, often called dental plaque, constantly form and grow on all tooth surfaces. Although production of acid by the bacteria in oral biofilms is the direct cause of dental caries, it is noteworthy that increases in the proportions of aciduric organisms, such as the caries-associated Streptococcus mutans, appear to occur at the expense of species that are less acid tolerant (i.e. less “aciduric”). Of particular note, a subset of less aciduric organisms derives protection from plaque acidification by alkali generation, which shows a positive association with dental health.
One of the primary routes for alkali generation by oral bacteria is the arginine deiminase system (ADS), through which arginine is catabolized into ornithine, ammonia and CO2, with the concomitant generation of ATP. Hence, the ADS serves key physiological functions in bacteria, providing protection from the deleterious effects of low pH and ATP for growth and maintenance. The ADS activity in oral biofilms can impact the ecology of oral microbial communities by moderating the pH through ammonia production.
A variety of bacteria that colonize the teeth and oral soft tissues and form oral biofilms express the ADS. An increased risk for dental caries has been associated with a reduced ability of oral biofilms to produce alkali from arginine via the arginine deiminase system (ADS). Specifically, plaque bacteria from caries-free subjects present higher levels of ADS activity when compared to plaque bacteria from caries-active subjects. Moreover, there is a high degree of variability in the rate of alkali production among individuals, in some cases greater than 1000-fold. A better understanding of the microbiological basis of inter-subject variation in ADS activity and methods for improving ADS activity for the improvement of oral health would be beneficial.
Other streptococcal species can also produce and secrete antimicrobial substances that suppress the growth of S. mutans, including hydrogen peroxide (H2O2). For example, Streptococcus sanguinis and Streptococcus gordonii produce sufficient amounts of H2O2 to inhibit growth of S. mutans, mainly via a pyruvate oxidase enzyme (Pox) encoded by the spxB gene. H2O2 production by S. gordonii, but not S. sanguinis, seems to be inversely correlated with carbohydrate availability. S. oligofermentans can also produce H2O2 from lactic acid through lactate oxidase (Lox) and less efficiently using L-amino acid oxidase enzymes (LAAO). Moreover, facultatively anaerobic lactic-acid bacteria, including commensal oral streptococci, were shown to have vigorous oxygen metabolism catalyzed by flavin-containing NADH oxidases, some of which yield H2O2 as an end product.